1. Field of the Invention
The present invention relates in general to an improved process for making thick, transparent, crystalline oxide films and in particular to ferroelectric oxides such as lead titanate (PT:PbTiO.sub.3), lead zirconate (PZ:PbZrO.sub.3), lead zirconate titanate (PZT:PbZr.sub.x Ti.sub.1-x O.sub.3, 0&lt;x&lt;1), PZTs doped with one or two metals such as lanthanum (PLZT:Pb.sub.1-x La.sub.x (Zr.sub.y Ti.sub.(1-y)).sub.( 1-x/4)O.sub.3, barium titanate (BT:BaTiO.sub.3), strontium barium niobate (SBN:Sr.sub.x Ba.sub.1-x Nb.sub.1 O.sub.3), and lithium niobate (LB: LiNb.sub.1 O.sub.3).
2. Description of the Prior Art
The challenge in making films specifically for optical devices lies in obtaining transparent, crystalline films with the rhombohedral or tetragonal perovskite crystal structure required to support ferroelectric domain formation. In the past, well-formed 0.5 to 5 .mu.m thick transparent crystalline films were difficult to obtain due to uncontrolled growth of crystal grains and film cracking during annealing. Cracking occurs when the polymer network becomes very rigid from crystallization while simultaneously undergoing shrinkage due to loss of organic by-products.
Oxide films have been deposited previously from alcoholic solutions of metallo-organic (alkoxide and carboxylic acid salt) precursors using a sol-gel process. Those films were primarily thin conductive coatings for window glass. That Process calls specifically for gradually heating the coated substrate to 650.degree. C. which teaches contrary to the present Process. While uncontrolled grain growth is expected to occur in that sol-gel process, control of the crystallinity is not important when the object of the process is to produce thin conductive crystalline films.
Sol-gel deposition of PT, PZT and PLZT films has been previously reported by several sources In those techniques, metallo-organic starting materials are mixed in an organic solution. Water can be added to hydrolyze the solution to form inorganic polymers consisting of metal-oxygen-metal linkages. Hydrolysis can be catalyzed by either acid or base. In analogy with sol-gel production of silica-glasses, other additives can be used to affect the structure of the Polymer (or gel) in order to control the drying and consolidation of the gel to form the oxide.
The transparency of ferroelectric films deposited by the sol-gel process also depends on the chemical composition of the film. When PLZT films are made, uncontrolled grain growth results in lower transparency in zirconia-rich compositions. The MOSD process controls the grain growth resulting in more transparent zirconia-rich PLZT films.
These descriptions of sol-gel deposition of ferroelectrics in the literature do not address control of the grain structure of the film that is essential for making transparent ceramic films. In the present process, we rapidly consolidate or densify the film at moderate temperatures in an amorphous state. In this state the inorganic polymer is flexible enough to release stress and avoid cracking. A separate annealing step is used to crystallize the film. Our densification and annealing process is an advance of the state-of-the-art in making thick transparent, crystalline, ferroelectric PZT and PLZT.
It is the object of this invention to provide a process that controls grain growth in ferroelectric oxide films resulting in thick (0.5 .mu.m to 5 .mu.m), transparent, ferroelectric oxide films.